1. Field of the Invention
This invention relates to a variable air-gap type driving device for obtaining a low speed and high torque output required to drive, for example, a joint of an industrial robot.
2. Description of the Prior Art
FIG. 1 is a sectional view showing a conventional variable air-gap type driving device disclosed in, for example, Published Unexamined Patent Application No. 264559 of 1989. FIG. 2 (a) is a sectional view taken on line F--F of FIG. 1; and FIG. 2 (b) is a sectional view taken on line G--G of FIG. 7.
In FIG. 1 and 2, reference numeral 1 denotes a casing of the variable air-gap type driving device. Numeral 2 denotes a ring-shaped armature fixed at the inner peripheral surface of the casing 1. Numeral 3 denotes coils. Numeral 4 denotes an internal gear fixed at the inner peripheral surface of the casing 7. Numeral 5 denotes a rotor revolved by the electromagnetic attractive force generated by the armature 2. Numeral 6 denotes an external gear, linked with the rotor 5, revolving around the inner peripheral surface with the revolution of the rotor 5. Numeral 7 denotes plural cranks whose eccentric shafts 7a is revolutionarily supported by the rotor 5 and the external gear 6 through the bearings 9-11, and whose rotation shafts 7b are linked with the output shaft 14 through the bearings 12 and 13, and further which transmit only the autorotation of the rotor 5 to the output shaft 14 by the revolution of the eccentric shafts 7a (the rotation of the rotation shafts 7b) with the revolution of the rotor 5. Numeral 15 denotes bearings rotatively supporting the rotation axes 7b.
Now, the operation of the conventional variable air-gap type driving device will be described.
First of all, magnetic attractive force is generated by the armature 2 by sequential power supplement to the coils 3, then the rotor 5 revolves by the magnetic attractive force, autorotating.
Because the rotor 5 is linked with the external gear 6, the rotor 5 autorotates by one tooth of the external gear 6 every revolution of the rotor 5.
Namely, because the external gear 6 is equipped with fewer external teeth at least one than the internal teeth of the internal gear 4, and because the internal teeth of the internal gear 4 and the external teeth of the external gear 6 engage one by one with the revolution of the external gear 6 inside the internal gear 4, the external gear 6 autorotates by one tooth every revolution. (Autorotation naturally changes by changing the number of the teeth of the external gear 6.)
Accordingly, because the eccentric shafts 7a of the cranks 7 revolve with the revolution of the rotor 5 (external gear 6), the rotation shafts 7b transmit only the autorotation of the rotor 5 to the output shaft 14. Then the output shaft 14 rotates in a low speed and high torque state.
Because the conventional variable air-gap type driving device is composed as mentioned above, the conventional device requires three cranks to take out only the autorotation from the revolution movement of the rotor 5 to the output shaft 14. Therefore, the conventional device has held such a problem that it requires many parts bringing about being complicated in structure and large in size.